1. Field of the Invention
This invention relates to a method for producing a grain-oriented silicon steel sheet, especially producing a general purpose grain-oriented silicon steel sheet having good magnetic properties with a high production performance and few cracks, if any.
2. Description of the Related Art
Grain-oriented silicon steel sheets are mainly used for iron core materials in electrical components such as transformers. It is important that they have a high magnetic flux density and low iron loss. Therefore, complex production steps are used. Hot rolling is applied to a silicon steel slab having a thickness of 100 to 300 mm, after heating the slab at a higher temperature than that applied to common steel, once or more steps of cold rolling with intermediate annealing to adjust to the final thickness of the sheet, and applying decarbonization annealing followed by finish annealing after coating the sheet with an annealing separator for the purpose of obtaining secondary recrystallized grains and purification.
It is important, for improving magnetic properties, to allow crystal grains to grow along the {110}&lt;001&gt; direction (Goss orientation), i.e. to align the &lt;001&gt; axis--an axis of easy magnetization--along the rolling axis in the secondary recrystallized grains during finish annealing. The complex process as described above is especially adapted to produce a steel sheet having a microstructure of secondary recrystallized grains highly aligned with Goss orientation.
For the purpose of enhancing growth of secondary recrystallized grains, it is important to apply a dispersion phase called an inhibitor that suppresses the growth of the primary recrystallized grains along directions other than the Goss orientation. The inhibitor is applied to the steel in a uniform and appropriate size. The inhibitor has limited solubility in steels and includes sulfides, selenides and nitrides, representative examples being MnS, MnSe and AlN.
For finely dispersing these important inhibitors such as sulfides, selenides and nitrides in appropriate sizes, a conventional method has been used in which inhibitors are allowed to precipitate during hot rolling after completely dissolving the inhibitors by heating the slab prior to hot rolling. The slab heating temperature for sufficiently forming a solid solution of inhibitors is about 1400.degree. C., which is about 200.degree. C. higher than that for heating common steel slabs. While heating the slabs at such a high temperature is essential for this purpose, it causes undesirable results as follows:
(1) The energy cost per unit weight of slabs is high because the slab is heated to a high temperature.
(2) Molten scale tends to be generated, and hanging of slabs is often encountered.
(3) The surfaces of the slabs are over-decarbonized.
While an induction heater for the exclusive use of crude grain oriented silicon steels was developed and used for heating the slab to solve problems (2) and (3) described above, another problem still remained: increase of energy consumption.
Energy should be saved as much as possible for producing grain-oriented silicon steel sheet in high performance. Accordingly, reduction of energy consumption for heating slabs is an urgent problem. Apart from the high grade grain-oriented silicon steel sheets, reduction of production cost is especially important in common products having medium grades of magnetic properties. Therefore, reducing the energy required in heating the slab (i.e., lowering the heating temperature) is very advantageous.
Many investigators have endeavored to lower the heating temperature of the slab in producing grain-oriented silicon steel sheets. Among many results that have been disclosed, Japanese Examined Patent Publication 54-24685 discloses reducing the temperature of heating slabs to 1050.degree. to 1350.degree. C. by allowing elements such as As, Bi, Pb and Sb that segregate in grain boundaries to remain in the steel, to utilize them as inhibitors. Japanese Unexamined Patent Publication No. 57-158322 discloses slabs heated at a lower temperature by reducing the content of Mn in the steel to adjust the Mn/S ratio to 2.5 or less, as well as stabilizing secondary recrystallized grains by adding Cu. In Japanese Unexamined Patent Publication No. 57-89433, the temperature for heating slabs is reduced to as low as 1000.degree. to 1250.degree. C. by controlling both the ratio of columnar crystals in the slab and the reduction in secondary cold rolling using a slab containing such elements as S, Se, Sb, Bi, Pb, Sn and B besides Mn.
These processes were developed under the impression that AlN having an extremely low solubility in the steel might not be used as an inhibitor. In those processes the magnetic properties were not always satisfactory because of poor suppressing ability as an inhibitor. Further, in many cases, the process can only be practiced on a laboratory scale.
Although Japanese Unexamined Patent Publication No. 59-190324 discloses pulse annealing applied for annealing the primary recrystallized grains, this art applies only to work in laboratories.
Japanese Unexamined Patent Publication 59-56522 discloses a method in which the temperature for heating slabs is decreased by adjusting the contents of Mn to 0.08 to 0.45% and of S to 0.007% or less and, in Japanese Unexamined Patent Publication 59-190325, Cr is added to the above composition for attempting to stabilize the secondary recrystallized grains. Both references are characterized by attempting to form a solid solution of MnS during heating of the slab by decreasing the percentage of S. In the case of slabs having a large mass, there arose a problem that magnetic properties along the transverse and longitudinal directions were not uniformly distributed.
A combined art of extremely low carbonization in silicon steels (C: 0.002 to 0.010%) and heating slabs at low temperature is disclosed in Japanese Unexamined Patent Publication 57-207114. This art is based on the belief that hot rolling when the temperature for heating the slab is low is advantageous for the subsequent formation of secondary recrystallized grains because the slabs do not undergo the austenite phase during coagulation. While such extremely low content of C is advantageous for preventing cracks from appearing during cold rolling, nitriding is required during decarbonization annealing for the purpose of stabilizing secondary recrystallized grains.
Once the art according to Japanese Unexamined Patent Publication 57-207114 described above has been disclosed, developments of nitriding during the production process increased. For example, an art enabling the operator to lower the temperature for heating the slab is disclosed in Japanese Unexamined Patent Publication 62-70521, wherein the conditions for finish annealing are specified and nitriding is carried out on the way of finish annealing. Further, in Japanese Unexamined Patent Publication 62-40315, a method is disclosed in which inhibitors are controlled at a proper level by a nitriding on the way of processing after adding a prescribed amount of Al and N unable to form a solid solution in the slab during heating.
However, such nitriding creates a new problem that additional facilities are required, hence increasing the cost; further, controlling nitriding on the way of finish annealing is difficult to control.